The present invention relates to methods and apparatus for electro-optically reading symbols, for example, bar code symbols and, in preferred embodiments, to compact hand held or hand worn laser beam scanners.
Bar code readers are known in the prior art for reading various symbologies such as UPC bar code symbols appearing on a label or on the surfaces of an article. The bar code symbol itself is a coded pattern of indicia comprised of a series of bars of various widths spaced apart from one another to bound spaces of various widths, the bars and spaces having different light reflecting characteristics. The readers in scanning or imaging systems electro-optically transform the graphic indicia into electrical signals, which are decoded into information, typically descriptive of the article or some characteristic thereof. Such information is conventionally represented in digital form and used as an input to a data processing system for applications in point-of-sale processing, inventory control and the like. Scanning systems of this general type have been disclosed, for example, in U.S. Pat. No. 5,600,121, assigned to the same assignee as the instant application. Such systems may employ a hand held, portable laser scanning device held by a user, which is configured to allow the user to aim the device, and more particularly, a light beam, at a targeted symbol to be read.
The light source in a laser scanner bar code reader is typically a semiconductor laser. The use of semiconductor devices as the light source is especially desirable because of their small size, low cost and low voltage requirements. The laser beam is optically modified, typically by an optical assembly, to form a beam spot of a certain size at the target distance. It is preferred that the cross section of the beam spot at the target distance be approximately the same as the minimum width between regions of different light reflectivity, i.e., the bars and spaces of the symbol.
In the laser beam scanning systems known in the art, the laser light beam is directed by a lens or other optical components along the light path toward a target that includes a bar code symbol on the surface. The moving-beam scanner operates by repetitively scanning the light beam in a line, pattern or series of lines across the symbol by means of motion of a scanning component, such as the light source itself or a mirror disposed in the path of the light beam. The scanning component may either sweep the beam spot across the symbol and trace a scan line across the pattern of the symbol, or scan the field of view of the scanner, or both.
Bar code reading systems also include a sensor or photo detector which detects light reflected or scattered from the symbol. The photo detector or sensor is positioned in the scanner in an optical path so that it has a field of view which ensures the capture of a portion of the light which is reflected or scattered off the symbol. This light is detected and converted into an electrical signal.
Some bar code reading systems are xe2x80x9cretro-reflectivexe2x80x9d. In a retro-reflective system, a moving mirror is used to transmit the outgoing beam and receive reflected light. Non-retro-reflective systems typically employ a moving mirror to transmit the outgoing beam and a separate detection system with a wide, static field of view.
Electronic circuitry and software decode the electrical signal into a digital representation of the data represented by the symbol that has been scanned. For example, the analog electrical signal generated by the photo detector is converted by a digitizer into a pulse or modulated digitized signal, with the widths corresponding to the physical widths of the bars and spaces. Such a digitized signal is then decoded, based on the specific symbology used by the symbol, into a binary representation of the data encoded in the symbol, and subsequently to the information or alphanumeric characters so represented.
The decoding process of known bar code reading system usually works in the following way. The decoder receives the pulse width modulated digitized signal from the digitizer, and an algorithm, implemented in the software, attempts to decode the signal. If the start and stop characters and information between them in the scan were decoded successfully, the decoding process terminates and an indicator of a successful read (such as a green light and/or an audible beep) is provided to the user. Otherwise, the decoder receives the next scan, performs another decode attempt on that scan, and so on, until a satisfactorily decoded scan is achieved or no more scans are available.
Such a signal is then decoded according to the specific symbology into a binary representation of the data encoded in the symbol, and to the information or alphanumeric characters so represented. The decoded information may be stored or subjected to data processing.
Hand held bar code scanners typically used today employ a gun-shaped housing having a handle portion and a head portion which contains the scanning module. Such systems may have a volume of 25 to 50 cubic inches. Smaller bar code scanners have been proposed such as the ring scanner shown in U.S. Pat. No. 5,543,610, pen mounted systems shown in U.S. Pat. No. 5,874,722 and U.S. application Ser. No. 08/794,782 filed Feb. 3, 1997, which is hereby incorporated by reference and the compact, elongated bar shaped scanner shown in U.S. application Ser. Nos. 09/166,816 filed Oct. 5, 1998 and 09/467,905 filed Dec. 21, 1999, both of which are assigned to Symbol Technologies, Inc., and both of which are hereby incorporated by reference. Implementation of smaller scanners requires a more compact scanning module.
Accordingly, it is a general object of the present invention to provide a more compact and capable bar code scanning module.
Typically, scanning modules employ a moving scan mirror. The mirror is mounted to pivot about one or more axes of rotation and driven by a magnet/coil assembly. In conventional scanning modules, the mirror is mounted on a leaf spring, for example, of mylar, plastic or metal. In such an arrangement the leaf spring is anchored at one end to housing or chassis of the scanning module and a mirror is attached at the other end. When driven, the mirror experiences a large deflection which produces the scanning action. It has also been proposed in U.S. Pat. No. 5,422,471 to Plesko to employ one or more spiral springs to provide a restoring force in a moving scanning mirror system. See FIGS. 12 through 17 of the ""471 patent, showing spiral, wire springs.
With the development of hand held scanner wands, pen scanners and ring scanners, there is a need for more compact, robust and stable moving mirror assemblies which consume relatively little power.
Accordingly, it is an object of the present invention to provide a compact scanning mirror assembly.
It is another object of the present invention to provide a scanning mirror assembly which is resistant to g-force effects.
It is another object of the present invention to provide a scanning mirror assembly with lower power requirements and sharper resonance characteristics.
It is another object of the present invention to provide a scanning mirror assembly with an accurately controllable scanning motion.
It has been proposed to use micro machined motors to provide small moving mirror structures. Such structures are described in U.S. patent application Ser. No. 09/035,763, entitled xe2x80x9cControl System For Oscillating Optical Elements In Scannersxe2x80x9d to Giebel et al., assigned to Symbol Technologies, Inc. and hereby incorporated by reference. Applicants have observed that the reduction in the size of scanning modules, particularly the use of smaller moving mirror structures, smaller detectors and smaller laser diode assemblies necessitates improvements in the construction and integration of these components to avoid degradation of the performance of the scanning system.
Accordingly, it is an object of the present invention to provide a compact scanning system which accurately and reliably reads bar code symbols.
It is a further object of the present invention to provide a robust and inexpensively fabricated, miniature bar code scanner module with performance similar to larger systems.
These and other objects and features will be apparent from the following summary and descriptions of preferred embodiments.
The present invention relates to techniques and assemblies for miniaturizing bar code scanners and modules. Preferred embodiments employ a semiconductor light source for generating a laser beam projected along a first optical path to a symbol to be read. A generally planar, reciprocally oscillatable reflector is located in the first optical path, for directing the laser beam impinging on the reflector to the symbol. The reflector is supported for reciprocal oscillating movement. A drive system reciprocally oscillates the reflector. A photo detector generates an electrical signal indicative of the detected light intensity.
In preferred embodiments, the reflector is supported by a spiral tape spring with a radially outer portion attached to the reflector. More particularly, the scanning device may include an axial member, an optical component such as the mirror driven to pivot with respect to the axial member to provide a scan pattern, and a spiral spring, generally coaxial with the axial member. Advantageously, the spiral spring has an inner end portion fixed to the axial member and spirals outwardly from the axial member. The spiral spring supports the optical component for pivoting with respect to the axial member, the optical component being attached to the spiral spring at a location radially displaced from the fixed end portion of the spiral spring.
The scanning device may further include a laser for directing a light beam toward said optical component and an electromagnetic driver for driving the optical component to pivot including a magnet and a coil. A mounting bracket may be provided for attaching the optical component and the magnet to the spiral spring at the location radially displaced from the fixed end portion of the spiral spring.
The spiral spring may have a thickness T and a width W in the axial direction such that W is much greater than T. A second spiral spring may be provided for supporting said axial member to permit pivoting of the optical component about a second axis.
The oscillating optical component or reflector may be a plane mirror and the spiral spring tends to maintain the mirror plane parallel to the axis of the spiral spring. One or more stops may be provided, normally out of contact with the mirror, for preventing movement of the mirror beyond a predetermined limit. One or more of such stops may be employed to prevent telescoping of the spiral spring beyond a predetermined limit.
Another preferred embodiment of the present invention uses a vertical cavity surface emitting laser (xe2x80x9cVCSELxe2x80x9d) as its source of laser light. Though any type of moving mirror structure may be employed with a VCSEL light source, a micro machined motor may advantageously be used to reduce the size of the scanner.
Generally, to reduce the size of the scanning module, the size of its component parts must be reduced. In particular, the light collection area of the system may be reduced, due to reduction of the size of the moving mirror and/or reduction in the size of the detector element itself. This gives rise to increased xe2x80x9cspeckle noisexe2x80x9d. Speckle noise is a type of self interference which occurs when coherent light waves from the laser diode are scattered by irregularities in the surface bearing the bar code. The result is a granular pattern known as speckle noise which changes as the beam scans across the substrate. The changing speckle pattern gives rise to random changes in the electrical output of the photo diode, which may interfere with the detection of the bar code symbol.
The effect of speckle noise reduced by use of the following techniques or devices, alone or in combination: shorter wavelength laser diodes e.g. less than 650 nm; a cylindrical optical element or diffractive optical element in the laser focusing system to produce an elliptical spot; a non-differentiating digitizer to avoid enhancing speckle noise in close range scanning; an offset correction circuit to prevent saturation when using a non-differentiating digitizer; and a multi-bit decoder and digital filtering schemes to filter out speckle noise.
In another preferred embodiment, speckle noise is reduced by using a scanning laser beam consisting of a small central spot of a size comparable to the smallest feature of the symbol being read and an annular halo, produced by the same laser diode, which is significantly larger than the smallest feature of the symbol, thereby averaging response from black and white areas located outside of the central spot. Advantageously, as much as a third and, preferably, as much as half of the power of the laser diode is contained in the halo.
The foregoing has been provided as a convenient summary of preferred embodiments. However, the invention to be protected is defined by the claims herein and the range of equivalents properly accorded thereto.